The 26{middle dot}5 ka Oruanui Eruption, Taupo Volcano, New Zealand: Development, Characteristics and Evacuation of a Large Rhyolitic Magma Body

The caldera-forming 26·5 ka Oruanui eruption (Taupo,New Zealand) erupted 530 km³ of magma, >99% rhyolitic, <1%mafic. The rhyolite varies from 71·8 to 76·7 wt% SiO₂ and 76 to 112 ppm Rb but is dominantly 74–76 wt% SiO₂. Average rhyolite compositions at each stratigraphiclevel do not change significantly through the eruption sequence.Oxide geothermometry, phase equilibria and volatile contentsimply magma storage at 830–760°C, and 100–200MPa. Most rhyolite compositional variations are explicable by28% crystal fractionation involving the phenocryst and accessoryphases (plagioclase, orthopyroxene, hornblende, quartz, magnetite,ilmenite, apatite and zircon). However, scatter in some elementconcentrations and ⁸⁷Sr/⁸⁶Sr ratios, and the presence of non-equilibriumcrystal compositions imply that mixing of liquids, phenocrystsand inherited crystals was also important in assembling thecompositional spectrum of rhyolite. Mafic compositions comprisea tholeiitic group (52·3–63·3 wt % SiO₂)formed by fractionation and crustal contamination of a contaminatedtholeiitic basalt, and a calc-alkaline group (56·7–60·5wt % SiO₂) formed by mixing of a primitive olivine–plagioclasebasalt with rhyolitic and tholeiitic mafic magmas. Both maficgroups are distinct from other Taupo Volcanic Zone eruptivesof comparable SiO₂ content. Development and destruction by eruptionof the Oruanui magma body occurred within 40 kyr and Oruanuicompositions have not been replicated in vigorous younger activity.The Oruanui rhyolite did not form in a single stage of evolutionfrom a more primitive forerunner but by rapid rejuvenation ofa longer-lived polygenetic, multi-age ‘stockpile’of silicic plutonic components in the Taupo magmatic system. KEY WORDS: Taupo Volcanic Zone; Taupo volcano; Oruanui eruption; rhyolite, zoned magma chamber; juvenile mafic compositions; eruption withdrawal systematics     

Leucocratic and Gabbroic Xenoliths from Hualalai Volcano, Hawai'i

A diverse range of crustal xenoliths is hosted in young alkalibasalt lavas and scoria deposits (erupted 3–5 ka) at thesummit of Huallai. Leucocratic xenoliths, including monzodiorites,diorites and syenogabbros, are distinctive among Hawaiian plutonicrocks in having alkali feldspar, apatite, zircon and biotite,and evolved mineral compositions (e.g. albitic feldspar, clinopyroxeneMg-number 67–78). Fine-grained diorites and monzodioritesare plutonic equivalents of mugearite lavas, which are unknownat Huallai. These xenoliths appear to represent melt compositionsfalling along a liquid line of descent leading to trachyte—amagma type which erupted from Huallai as a prodigious lava flowand scoria cone at 114 ka. Inferred fractionating assemblages,MELTS modeling, pyroxene geobarometry and whole-rock norms allpoint to formation of the parent rocks of the leucocratic xenolithsat 3–7 kbar pressure. This depth constraint on xenolithformation, coupled with a demonstrated affinity to hypersthene-normativebasalt and petrologic links between the xenoliths and the trachyte,suggests that the shift from shield to post-shield magmatismat Huallai was accompanied by significant deepening of the activemagma reservoir and a gradual transition from tholeiitic toalkalic magmas. Subsequent differentiation of transitional basaltsby fractional crystallization was apparently both extreme—culminatingin >5·5 km³ of trachyte—and rapid, at 2·75x 10⁶ m³ magma crystallized/year. KEY WORDS: geothermobarometry; magma chamber; xenolith; cumulate; intensive parameters     

Thermal History of the Horoman Peridotite Complex: A Record of Thermal Perturbation in the Lithospheric Mantle

The ascent history of the Horoman peridotite complex, Hokkaido,northern Japan, is revised on the basis of a detailed studyof large ortho- and clinopyroxene grains 1 cm in size (megacrysts)in the Upper Zone of the complex. The orthopyroxene megacrystsexhibit distinctive M-shaped Al zoning patterns, which werenot observed in porphyroclastic grains less than 5 mm in sizedescribed in previous studies. Moreover, the Al and Ca contentsof the cores of the orthopyroxene megacrysts are lower thanthose of the porphyroclasts. The Upper Zone is inferred to haveresided not only at a higher temperature than previously suggestedbut also at a higher pressure (1070°C, 2·3 GPa) thanthe Lower Zone (950°C, 1·9 GPa), in the garnet stabilityfield, before the ascent of the two zones. The Horoman complexprobably represents a 12 ± 5 km thick section of lithosphericmantle with an 10 ± 8°C/km vertical thermal gradient.The current thickness of the Horoman complex is 3 km, whichis a result of shortening of the lithospheric mantle by 0·25± 0·1 during its ascent. The Upper Zone appearsto have experienced a heating event during its ascent throughthe spinel stability field, with a peak temperature as highas 1200°C. The effect of heating decreases continuouslytowards the base of the complex, and the lowermost part of theLower Zone underwent very minor heating at a pressure higherthan 0·5 GPa. The uplift and associated deformation,as well as heating, was probably driven by the ascent of a hotasthenospheric upper-mantle diapir into the Horoman lithosphere. KEY WORDS: Horoman; P–T trajectory; thermal history; Al diffusion in pyroxene; geothermobarometry     

Early-Middle Jurassic Dolerite Dykes from Western Dronning Maud Land (Antarctica): Identifying Mantle Sources in the Karoo Large Igneous Province

A suite of dolerite dykes from the Ahlmannryggen region of westernDronning Maud Land (Antarctica) forms part of the much moreextensive Karoo igneous province of southern Africa. The dykecompositions include both low- and high-Ti magma types, includingpicrites and ferropicrites. New ⁴⁰Ar/³⁹Ar age determinationsfor the Ahlmannryggen intrusions indicate two ages of emplacementat 178 and 190 Ma. Four geochemical groups of dykes have beenidentified in the Ahlmannryggen region based on analyses of60 dykes. The groups are defined on the basis of whole-rockTiO₂ and Zr contents, and reinforced by rare earth element (REE),⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope data. Group 1 were intrudedat 190 Ma and have low TiO₂ and Zr contents and a significantArchaean crustal component, but also evidence of hydrothermalalteration. Group 2 dykes were intruded at 178 Ma; they havelow to moderate TiO₂ and Zr contents and are interpreted tobe the result of mixing of melts derived from an isotopicallydepleted source with small melt fractions of an enriched lithosphericmantle source. Group 3 dyke were intruded at 190 Ma and formthe most distinct magma group; these are largely picritic withsuperficially mid-ocean ridge basalt (MORB)-like chemistry (flatREE patterns, ⁸⁷Sr/⁸⁶Sr_i 0·7035, Nd_i 9). However, theyhave very high TiO₂ (4 wt %) and Zr (500 ppm) contents, whichis not consistent with melting of MORB-source mantle. The Group3 magmas are inferred to be derived by partial melting of astrongly depleted mantle source in the garnet stability field.This group includes several high Mg–Fe dykes (ferropicrites),which are interpreted as high-temperature melts. Some Group3 dykes also show evidence of contamination by continental crust.Group 4 dykes are low-K picrites intruded at 178 Ma; they havevery high TiO₂–Zr contents and are the most enriched magmagroup of the Karoo–Antarctic province, with ocean-islandbasalt (OIB)-like chemistry. Dykes of Group 1 and Group 3 aresub-parallel (ENE–WSW) and both groups were emplaced at190 Ma in response to the same regional stress field, whichhad changed by 178 Ma, when Group 2 and Group 4 dykes were intrudedalong a dominantly NNE–SSW strike. KEY WORDS: flood basalt; depleted mantle; enriched mantle; Ahlmannryggen; Karoo dyke     

Correlations between U, Th Content and Metamorphic Grade in the Western Namaqualand Belt, South Africa, with Implications for Radioactive Heating of the Crust

The digital image of airborne radiometric data across SouthAfrica reveals that the largest anomaly, 100 nGy/h, is causedby the granulite-facies rocks of the Namaquan metamorphic complex,whereas most of the country is <60 nGy/h. This observationis consistent with geochemical data that show that the 1900± 100 Ma greenschist-facies Richtersveld Terrane nearNamibia (max. U = 3·4 ppm; Th = 20·1 ppm) andthe adjacent, 1100 ± 100 Ma, amphibolite-facies Aggeneys/SteinkopfTerranes (max. U 10 ppm; Th 52 ppm) are the least enrichedin U, Th and K. In contrast, the lower-T granulite-facies OkiepTerrane near Springbok hosts more enriched granites (max. U 17 ppm; Th 66 ppm) and noritic intrusions (max. U = 14 ppm;Th = 83 ppm). The most enriched rocks are found in the 1030Ma higher-T granulite-facies core of the Namaquan belt and includequartzo-feldspathic gneisses (max. U = 46 ppm; Th = 90 ppm)and charnockites (max. U = 52 ppm; Th = 400 ppm). Our findingscontradict the notion that granulite-facies terrains are characteristicallydepleted in U and Th. In this study we modeled the heat productionin the core of the Namaquan complex, where the granulites havehad a very unusual metamorphic history, and show that ultra-high-T(1000°C, P 10 kbar) metamorphic conditions could have beenachieved by radiogenic heating without invoking external heatsources. However, monazite-rich veins of charnockite and patchesof granulites mark the passage of CO₂-dominated melts and fluidsderived from fractionated noritic intrusions. KEY WORDS: charnockite; granulite; Namaqualand; thorium; uranium; radioactive heating; metamorphism     

Rates of Thermal and Chemical Evolution of Magmas in a Cooling Magma Chamber: a Chronological and Theoretical Study on Basaltic and Andesitic Lavas from Rishiri Volcano, Japan

Rates of magmatic processes in a cooling magma chamber wereinvestigated for alkali basalt and trachytic andesite lavaserupted sequentially from Rishiri Volcano, northern Japan, bydating of these lavas using ²³⁸U–²³⁰Th radioactive disequilibriumand ¹⁴C dating methods, in combination with theoretical analyses.We obtained the eruption age of the basaltic lavas to be 29·3± 0·6 ka by ¹⁴C dating of charcoals. The eruptionage of the andesitic lavas was estimated to be 20·2 ±3·1 ka, utilizing a whole-rock isochron formed by U–Thfractionation as a result of degassing after lava emplacement.Because these two lavas represent a series of magmas producedby assimilation and fractional crystallization in the same magmachamber, the difference of the ages (i.e. 9 kyr) is a timescaleof magmatic evolution. The thermal and chemical evolution ofthe Rishiri magma chamber was modeled using mass and energybalance constraints, as well as quantitative information obtainedfrom petrological and geochemical observations on the lavas.Using the timescale of 9 kyr, the thickness of the magma chamberis estimated to have been about 1·7 km. The model calculationsshow that, in the early stage of the evolution, the magma cooledat a relatively high rate (>0·1°C/year), and thecooling rate decreased with time. Convective heat flux fromthe main magma body exceeded 2 W/m² when the magma was basaltic,and the intensity diminished exponentially with magmatic evolution.Volume flux of crustal materials to the magma chamber and rateof convective melt exchange (compositional convection) betweenthe main magma and mush melt also decreased with time, from 0·1 m/year to 10^–3 m/year, and from 1 m/yearto 10^–2 m/year, respectively, as the magmas evolved frombasaltic to andesitic compositions. Although the mechanism ofthe cooling (i.e. thermal convection and/or compositional convection)of the main magma could not be constrained uniquely by the model,it is suggested that compositional convection was not effectivein cooling the main magma, and the magma chamber is consideredto have been cooled by thermal convection, in addition to heatconduction. KEY WORDS: convection; magma chamber; heat and mass transport; timescale; U-series disequilibria     

Origin of Paleoproterozoic Komatiites at Jeesiorova, Kittila Greenstone Complex, Finnish Lapland

Komatiites from the 2 Ga Jeesiörova area in Finnish Laplandhave subchondritic Al₂O₃/TiO₂ ratios like those in Al-depletedkomatiites from Barberton, South Africa. They are distinct inthat their Al abundances are higher than those of the Al-depletedrocks and similar to levels in Al-undepleted komatiites. Moderatelyincompatible elements such as Ti, Zr, Eu, and Gd are enriched.Neither majorite fractionation nor hydrous melting in a supra-subductionzone setting could have produced these komatiites. Their highconcentrations of moderately incompatible elements may haveresulted from contamination of their parental melt through interactionwith metasomatic assemblages in the lithospheric mantle or enrichmentof their mantle source in basaltic melt components. Re–Osisotope data for chromite from the Jeesiörova rocks yieldan average initial ¹⁸⁷Os/¹⁸⁸Os of 0·1131 ± 0·0006(2), _Os(I) = 0·1 ± 0·5. These data, coupledwith an initial _Nd of +4, indicate that melt parental to thekomatiites interacted minimally with ancient lithospheric mantle.If their mantle source was enriched in a basaltic component,the combined Os–Nd isotopic data limit the enrichmentprocess to within 200 Myr prior to the formation of the komatiites.Their Os–Nd isotopic composition is consistent with derivationfrom the contemporaneous convecting upper mantle. KEY WORDS: Finnish Lapland; Jeesiörova; komatiites; mantle geochemistry; petrogenesis; redox state; Re/Os isotopes; Ti enrichment     

Geochemistry of South African On- and Off-craton, Group I and Group II Kimberlites: Petrogenesis and Source Region Evolution

Bulk-rock geochemical compositions of hypabyssal kimberlites,emplaced through the Archaean Kaapvaal craton and ProterozoicNamaqua–Natal belt, are used to estimate close-to-primarymagma compositions of Group I kimberlites (Mg-number = 0·82–0·87;22–28 wt % MgO; 21–30 wt % SiO₂; 10–17 wt% CaO; 0·2–1·7 wt % K₂O) and Group II kimberlites(Mg-number = 0·86–0·89; 23–29 wt %MgO; 28–36 wt % SiO₂; 8–13 wt % CaO; 1·6–4·6wt % K₂O). Group I kimberlites are distinguished from GroupII by their lower Ba/Nb (<12), Th/Nb (<1·1) andLa/Nb (<1·1) but higher Ce/Pb (>22) ratios. Thedistinct rare earth element patterns of the two types of kimberlitesindicate a more highly metasomatized source for Group II kimberlites,with more residual clinopyroxene and less residual garnet. Thesimilarity of Sr and Nd isotope ratios and diagnostic traceelement ratios (Ce/Pb, Nb/U, La/Nb, Ba/Nb, Th/Nb) of Group Ikimberlites to ocean island basalts (OIB), but more refractoryMg-numbers and Ni contents, are consistent with derivation ofGroup I kimberlites from subcontinental lithospheric mantle(SCLM) that has been enriched by OIB-like melts or fluids. Sourceenrichment ages and plate reconstructions support a direct associationof these melts or fluids with Mesozoic upwelling beneath southernAfrica of a mantle plume(s), at present located beneath thesouthern South Atlantic Ocean. In contrast, the geochemicalcharacteristics of both on- and off-craton Group II kimberlitesshow strong similarity to calc-alkaline magmas, particularlyin their Nb and Ta depletion and Pb enrichment. It is suggestedthat Group II kimberlites are derived from both Archaean andProterozoic lithospheric mantle source regions metasomatizedby melts or fluids associated with ancient subduction events,unrelated to mantle plume upwelling. The upwelling of mantleplumes beneath southern Africa during the Mesozoic, at the timeof Gondwana break-up, may have acted as a heat source for partialmelting of the SCLM and the generation of both Group I and GroupII kimberlite magmas. KEY WORDS: kimberlite; geochemistry; petrogenesis; mantle plumes; South Africa     

The Origin and Evolution of the Kaapvaal Cratonic Lithospheric Mantle

A detailed petrological and geochemical study of low-temperatureperidotite xenoliths from Kimberley and northern Lesotho ispresented to constrain the processes that led to the magmaphileelement depletion of the Kaapvaal cratonic lithospheric mantleand its subsequent re-enrichment in Si and incompatible traceelements. Whole-rocks and minerals have been characterized forRe–Os isotope compositions, and major and trace elementconcentrations, and garnet and clinopyroxene for Lu–Hfand Sm–Nd isotope compositions. Most samples are characterizedby Archaean Os model ages, low Al, Fe and Ca contents, highMg/Fe, low Re/Os, very low (< 0·1 x chondrite) heavyrare earth element (HREE) concentrations and a decoupling betweenNd and Hf isotope ratios. These features are most consistentwith initial melting at 3·2 Ga followed by metasomatismby hydrous fluids, which may have also caused additional meltingto produce a harzburgitic residue. The low HREE abundances ofthe peridotites require that extensive melting occurred in thespinel stability field, possibly preceded by some melting inthe presence of garnet. Fractional melting models suggest that30% melting in the spinel field or 20% melting in the garnetfield followed by 20% spinel-facies melting are required toexplain the most melt-depleted samples. Garnet Nd–Hf isotopecharacteristics indicate metasomatic trace element enrichmentduring the Archaean. We therefore suggest a model includingshallow ridge melting, followed by metasomatism of the Kaapvaalupper mantle in subduction zones surrounding cratonic nuclei,probably during amalgamation of smaller pre-existing terranesin the Late Archaean (2·9 Ga). The fluid-metasomatizedresidua have subsequently undergone localized silicate meltinfiltration that led to clinopyroxene ± garnet enrichment.Calculated equilibrium liquids for clinopyroxene and their Hf–Ndisotope compositions suggest that most diopside in the xenolithscrystallized from an infiltrating kimberlite-like melt, eitherduring Group II kimberlite magmatism at 200–110 Ma (Kimberley),or shortly prior to eruption of the host kimberlite around 90Ma (northern Lesotho). KEY WORDS: Kaapvaal craton; lithospheric mantle; metasomatism; Nd–Hf isotopes; Re–Os isotopes     

Zircon Hf Isotopic Evidence for Mixing of Crustal and Silicic Mantle-derived Magmas in a Zoned Granite Pluton, Eastern Australia

Zircon Hf isotopic data from a zoned pluton of the Moonbi supersuite,New England batholith, eastern Australia, are consistent withmagma mixing between two silicic melts, each derived from isotopicallydistinct sources. Although zircons from three zones within theWalcha Road pluton give a U–Pb crystallization age of249 ± 3 Ma, zircon populations from each zone have arange in _Hf. Zircons from the mafic hornblende–biotitemonzogranite pluton margin and intermediate zones have _Hf +5to +11, whereas those from the more felsic centre of the plutonhave _Hf +7 to +16, representing a total variation of 11 _Hfunits. The Lu–Hf depleted mantle model ages range from650 to 250 Ma, with the younger zircons present only in thefelsic pluton centre. The variation in _Hf indicates the involvementof silicic melts from at least two sources, one a crustal componentwith a Neoproterozoic model age and the other a primitive mantle-derivedcomponent with model ages similar to the U–Pb crystallizationage of the pluton. The zircons reflect the isotopic compositionsof the different proportions of crustal-derived silicic melt,relative to mantle-derived silicic melt, between melt generationand final pluton construction. The Walcha Road pluton is consideredto have formed by incremental assembly of progressively morefelsic melt batches resulting from mixing, replenishment andcrystal–melt separation, with final pluton constructioninvolving mechanical concentration as zones of crystal mush.The zoned pluton and, more broadly, the Moonbi supersuite provideexamples of magma mixing by which the more silicic units havemore juvenile isotopic compositions as a result of increasingproportions of residual melt from basalt fractionation, relativeto crustal partial melt. KEY WORDS: Australia; granite magma mixing; zircon; zoned pluton; Hf isotopes     

Experimental Constraints on the Role of Garnet Pyroxenite in the Genesis of High-Fe Mantle Plume Derived Melts

The anhydrous phase relations of an uncontaminated (primitive),ferropicrite lava from the base of the Early Cretaceous Paraná–Etendekacontinental flood basalt province have been determined between1 atm and 7 GPa. The sample has high contents of MgO (14·9wt %), FeO^* (14·9 wt %) and Ni (660 ppm). Olivine phenocrystshave maximum Fo contents of 85 and are in equilibrium with thebulk rock, assuming a of 0·32. A comparison of our results with previous experimental studiesof high-Mg rocks shows that the high FeO content of the ferropicritecauses an expansion of the liquidus crystallization field ofgarnet and clinopyroxene relative to olivine; orthopyroxenewas not observed in any of our experiments. The high FeO contentalso decreases solidus temperatures. Phase relations indicatethat the ferropicrite melt last equilibrated either at 2·2GPa with an olivine–clinopyroxene residue, or at 5 GPawith a garnet–clinopyroxene residue. The low bulk-rockAl₂O₃ content (9 wt %) and high [Gd/Yb]_n ratio (3·1)are consistent with the presence of residual garnet in the ferropicritemelt source and favour high-pressure melting of a garnet pyroxenitesource. The garnet pyroxenite may represent subducted oceaniclithosphere entrained by the upwelling Tristan starting mantleplume head. During adiabatic decompression, intersection ofthe garnet pyroxenite solidus at 5 GPa would occur at a mantlepotential temperature of 1550°C and yield a ferropicriteprimary magma. Subsequent melting of the surrounding peridotiteat 4·5 GPa may be restricted by the thickness of theoverlying sub-continental lithosphere, such that dilution ofthe garnet pyroxenite melt component would be significantlyless than in intra-oceanic plate settings (where the lithosphereis thinner). This model may explain the limited occurrence offerropicrites at the base of continental flood basalt sequencesand their apparent absence in ocean-island basalt successions. KEY WORDS: continental flood basalt; ferropicrite; mantle heterogeneity; mantle melting; phase relations; pyroxenite     

Fluid--Rock Interaction during Low-Pressure Polymetamorphism of the Reynolds Range Group, Central Australia

Marbles and metapelites from the Reynolds Range Group (centralAustralia) were regionally metamorphosed at low pressure duringM₂ at 1.6 Ga, M₂ ranged in grade from greenschist to granulitefacies along the length of the Reynolds Range, and overprinted1.78 Ga granites and their contact aureoles in the ReynoldsRange Group metasediments. At all M₂ grades the marbles andmetapelites have highly variable oxygen isotope ratios [marbles:¹⁸O(carb) 14–20%; metapelites: ¹⁸O 6–14%). Similarly, 1.78 Ga granites have highly variable oxygen isotope ratios(¹⁸O 5–13%), with the lowest values occurring at thegranite margins. In all rock types, the lowest oxygen isotopevalues are consistent with the infiltration of channelled magmaticand/or meteoric fluids. The variable lowering of oxygen isotopevalues resulted from pre-M₂ contact metamorphism and fluid—rockinteraction around the 1.78 Ga granites. In contrast, mineralassemblages in the marbles define a trend of increasing X_CO2with increasing grade from <0.05 (greenschist facies) to0.7–1.0 (granulite facies). This, together with the lackof regionally systematic resetting of oxygen isotope ratios,implies that there was little fluid—rock interaction duringprograde regional metamorphism. KEY WORDS: low pressure; polymetamorphism; fluids; stable isotopes; petrology ^*Corresponding author Fax: 61–3–94791272. e-mail: geoisb{at}lure.latrobe.edu.au     

Dehydration--Melting Phenomena in Leptynitic Gneisses and the Generation of Leucogranites: a Case Study from the Kerala Khondalite Belt, Southern India

Pan-African high-grade metamorphism in the Kerala KhondaliteBelt (South India) led to the in situ formation of garnet-bearingleucosomes (L1) in sodic quartz—alkali feldspar—biotitegneisses. Microtextures, mineralogy and the geochemical characteristicsof in situ leucosomes (L1) and gneiss domains (GnD) indicatethat the development of leucosomes was mainly controlled bythe growth of garnet at the expense of biotite. This is documentedby the selective transfer of FeO, MgO, , Sm and the heavy rareearth elements into the L1 domains. P-T constraints (T>800C,P>6kbar, aH₂O0.3) suggest that the leucosomes were formedthrough complete melting of biotite in fluid-absent conditions,following the model reaction Biotite+Alkali feldspar+QuartzlGarnet+Ilmenite+Melt.The fraction of melt generated during this process was low (<10vol.%). The identical size of the leucosomes as well as theirhomogeneous and isotropic distribution at outcrop scale, whichlacks any evidence for melt segregation, suggest that the migmatiteremained a closed system. Subsequent to migmatization, the leptyniticgneisses were intruded by garnet-bearing leucogranitic melts(L2), forming veins parallel and subperpendicular to the foliation.The leucogranites are rich in potassium (K₂O5.5 wt%), (Ba400p.p.m.) and Sr (300 p.p.m.), and exhibit low concentrationsof Zr (40 p.p.m.), Th (<1 p.p.m.) and (<10 p.p.m.). Thechondrite-normalized REE spectra show low abundances (La_N20,Lu_N3) and are moderately fractionated (La_N/Lu_N7). An Eu anomalyis absent or weakly negative. The higher ⁸⁷Sr/⁸⁶Sr ratio at550 Ma (0.7345) compared with the migmatite (0.7164) precludesa direct genetic relationship between leptynitic gneisses andleucogranites at Manali.Nevertheless, the chemical and mineralogicalcompositions of the leuocogranites strongly favour a derivationthrough fluid-absent biotite melting of isotopically distinctbut chemically comparable Manali-type gneisses. The undersaturationof Zr, Th and REE, a typical feature of leucogranitic meltsgenerated during granulite facies anatexis of psammo-peliticlithologies and attributed to disequilibrium melting with incompletedissolution of accessory phases (zircon, monazite), is weaklydeveloped in the leucogranites of Manali.It is concluded thatthis is mainly due to the sluggish migration of the melts instatic conditions, which facilitated equilibration with therestitic gneisses. ^*Fax: 0228-732763; e-mail: ingo.braun{at}uni-bonn.de     

Comparison of Thermochronometers in a Slowly Cooled Granulite Terrain: Nagssugtoqidian Orogen, West Greenland

Uranium–Pb sphene and apatite, and ⁴⁰Ar/³⁹Ar hornblende,muscovite and K-feldspar ages from the core of the ProterozoicNagssugtoqidian orogen, West Greenland, are used to constrainthe timing of granulite-facies metamorphism and the subsequentcooling history. Metamorphic monazite growth occurred at 1858± 2, 1830 ± 1 and 1807 ± 2 Ma and definesthe peak of metamorphism. The uncertainty in the cooling rateshas to include the error in the decay constants of the systemsused. This source of uncertainty is, however, negligible ifa single decay scheme is used or when the age difference betweenthe chronometers is large (>100 m.y.). Over the last twodecades increasingly higher closure temperatures have been proposed.This trend reflects the difficulty of determining ‘absolute’closure temperatures and in using a limited number of closuretemperature estimates to infer closure temperatures of othergeochronometers. Cooling rates at Ussuit were 2·9 ±1·7°C/m.y. from 1762 Ma (670°C) to 1705 Ma (500°C),1·5 ± 1·1°C/m.y. from 1705 Ma to 1640Ma (410°C), and 0·9 ± 0·4°C/m.y.between 1640 and 1416 Ma (200°C). Between 1720 and 1645Ma cooling rates in Lersletten, 60 km north of Ussuit, are indistinguishablefrom those at Ussuit. After 1645 Ma, however, the area cooledto 200°C at a slightly faster rate of 2·6 ±1·2°C/m.y. KEY WORDS: ⁴⁰Ar/³⁹Ar and U–Pb geochronometers; granulite metamorphism; slow cooling; T–t path     

Staurolite Stability in a Part of the System Fe-Al-Si-O-H

The following reactions, believed to be analogous to those whichdefine the maximum extent of staurolite-quartz compatibilityat moderate oxygen fugacity in metamorphic rocks, have beendetermined in terms of hydrous fluid pressure and temperature.The O: H composition ratio of the fluid was controlled withthe quartz-fayalite-magnetite (QFM) buffer assemblage. (I) Fe-staurolitequartz almandine+sillimanite+water. (II) Fe-staurolitequartz Fe-cordierite+sillimanite+water. (III) Fe-chloritoid+sillimanite Fe-staurolite+quartz+water. In addition, two reactions which delineate part of the stabilitylimits of Fe-cordierite have been investigated: (IV) Fe-cordierite almandine+sillimanite+quartz. (V) Fe-cordierite hercynite+sillimanite+quartz. The experimental information has been used to predict boundariesto the PT fields of all quartz and QFM-buffered fluid-bearingassemblages involving Fe-staurolite, Fe-cordierite, Fe-chloritoid,almandine, and sillimanite. Using information from this andother studies, three mineral assemblages are recognized whichare stable at similar temperatures but different fluid pressures.In order of decreasing pressure they are: (a) Above 5 kb: staurolite, quartz, kyanite, fluid; (b) Between 1.5 and 8.5 kb (outer limits; in natural rocks thisfield will have a much narrower pressure range) staurolite,quartz, cordierite, fluid. (c) Below 3.5 kb: Fe-cordierite, andalusite, fluid of oxygenfugacity equivalent to the quartz-fayalite-magnetite assemblage. These phase assemblages may be the equivalents of naturallyoccurring mineral facies, but this must be proven in the field.In addition the absence of cordierite from rocks of appropriatecomposition and temperature of formation betokens total pressuresgreater than 3–5 kb. ¹Present address: Grant Institute of Geology, West Mains Road, Edinburgh 9, Scotland.     

The Gronnedal-Ika Carbonatite-Syenite Complex, South Greenland: Carbonatite Formation by Liquid Immiscibility

The Grønnedal-Ika complex is dominated by layered nephelinesyenites which were intruded by a xenolithic syenite and a centralplug of calcite to calcite–siderite carbonatite. Aegirine–augite,alkali feldspar and nepheline are the major mineral phases inthe syenites, along with rare calcite. Temperatures of 680–910°Cand silica activities of 0·28–0·43 weredetermined for the crystallization of the syenites on the basisof mineral equilibria. Oxygen fugacities, estimated using titanomagnetitecompositions, were between 2 and 5 log units above the fayalite–magnetite–quartzbuffer during the magmatic stage. Chondrite-normalized REE patternsof magmatic calcite in both carbonatites and syenites are characterizedby REE enrichment (La_CN–Yb_CN = 10–70). Calcite fromthe carbonatites has higher Ba (5490 ppm) and lower HREE concentrationsthan calcite from the syenites (54–106 ppm Ba). This isconsistent with the behavior of these elements during separationof immiscible silicate–carbonate liquid pairs. _Nd(T =1·30 Ga) values of clinopyroxenes from the syenites varybetween +1·8 and +2·8, and _Nd(T) values of whole-rockcarbonatites range from +2·4 to +2·8. Calcitefrom the carbonatites has ¹⁸O values of 7·8 to 8·6and ¹³C values of –3·9 to –4·6. ¹⁸Ovalues of clinopyroxene separates from the nepheline syenitesrange between 4·2 and 4·9. The average oxygenisotopic composition of the nepheline syenitic melt was calculatedbased on known rock–water and mineral–water isotopefractionation to be 5·7 ± 0·4. Nd and C–Oisotope compositions are typical for mantle-derived rocks anddo not indicate significant crustal assimilation for eithersyenite or carbonatite magmas. The difference in ¹⁸O betweencalculated syenitic melts and carbonatites, and the overlapin _Nd values between carbonatites and syenites, are consistentwith derivation of the carbonatites from the syenites via liquidimmiscibility. KEY WORDS: alkaline magmatism; carbonatite; Gardar Province; liquid immiscibility; nepheline syenite     

Stable Isotopic Evidence for Fluid-Rock Interactions in the Ivrea Zone, Italy

Stable isotope compositions of Ivrea Zone marbles and associatedlithologies are in general heterogeneous. The oxygen isotopecomposition of quartz in pelites ranges from ¹⁸O +9 to + 17(SMOW) and does not vary systematically with metamorphic grade.Peridotites retain oxygen isotope signatures close to mantlevalues. Marble calcites vary in isotopic composition from ¹³C + 2(PDB),¹⁸0 +24(SMOW)to ¹³C –6(PDB), ¹⁸O + 13 (SMOW).Depletions in ¹⁸O and ¹³C may be explained dominantly by interactionwith fluids derived from within the observed metasedimentarysequence during prograde metamorphism. ¹⁸O and ¹³C show gradients of greater than 5/m across marblemargins and within marbles. The preservation of such isotopicgradients is not consistent with the long-term presence of grain-boundary-scaleinterconnected fluid films in and around marbles. There is ageneral lowering of ¹⁸O within individual marble bodies althoughlarge carbon and oxygen isotopic gradients are present. Calcitein marbles may attain oxygen isotope equilibrium, but rarelycarbon isotope equilibrium, with surrounding metapelites. Infiltrationof marbles must involve a component of channelized fluid flow. The general lack of isotopic equilibration within the sequencerequires channelized fluid flow and limited fluid-rock ratios.Large pervasive mantle to crust fluid fluxes are not consistentwith the observations. ^*Present address: Natural Environment Research Council, Polaris House, North Star Avenue, Swindon SN2 1EU, England     

Magmatic Differentiation at an Island-arc Caldera: Okmok Volcano, Aleutian Islands, Alaska

Okmok volcano is situated on oceanic crust in the central Aleutianarc and experienced large (15 km³) caldera-forming eruptionsat 12 000 years BP and 2050 years BP. Each caldera-forming eruptionbegan with a small Plinian rhyodacite event followed by theemplacement of a dominantly andesitic ash-flow unit, whereaseffusive inter- and post-caldera lavas have been more basaltic.Phenocryst assemblages are composed of olivine + pyroxene +plagioclase ± Fe–Ti oxides and indicate crystallizationat 1000–1100°C at 0·1–0·2 GPain the presence of 0–4% H₂O. The erupted products followa tholeiitic evolutionary trend and calculated liquid compositionsrange from 52 to 68 wt % SiO₂ with 0·8–3·3wt % K₂O. Major and trace element models suggest that the moreevolved magmas were produced by 50–60% in situ fractionalcrystallization around the margins of the shallow magma chamber.Oxygen and strontium isotope data (¹⁸O 4·4–4·9,⁸⁷Sr/ ⁸⁶Sr 0·7032–0·7034) indicate interactionwith a hydrothermally altered crustal component, which led toelevated thorium isotope ratios in some caldera-forming magmas.This compromises the use of uranium–thorium disequilibria[(²³⁰Th/ ²³⁸U) = 0·849–0·964] to constrainthe time scales of magma differentiation but instead suggeststhat the age of the hydrothermal system is 100 ka. Modellingof the diffusion of strontium in plagioclase indicates thatmany evolved crystal rims formed less than 200 years prior toeruption. This addition of rim material probably reflects theremobilization of crystals from the chamber margins followingreplenishment. Basaltic recharge led to the expansion of themagma chamber, which was responsible for the most recent caldera-formingevent. KEY WORDS: Okmok; caldera; U-series isotopes; Sr-diffusion; time scales; Aleutian arc     

A Stable Isotope Study of Anorogenic Magmatism in East Central Asia

A stable isotope study of 168 plutonic igneous rock and fivewater samples from Transbaikalia, East Asia, has been made,including 318 whole-rock and mineral ¹⁸O/¹⁶O analyses and 14D/H analyses. This represents the first detailed isotopic studyof the enormous Phanerozoic K-rich (mostly anorogenic) granitoidbelts of this region, which are thousands of kilometers in lengthand span an age range of >250Ma. Of the five main intrusivesuites, emplacement of the youngest (Permo-Triassic) was accompaniedby intense meteoric-hydrothermal activity, locally causing extreme¹⁸O depletio (¹⁸O _feldspar<–12). This reflects thevery low ¹⁸O of the water involved in these systems, which probablyhad ¹⁸O < – 20 and D < – 150, consistent withthe high paleolatitude of Transbaikalia in the early Mesozoic(80N). Despite local post-emplacement, hydrothermal ¹⁸O-depletioneffects near Permo-Triassic and younger plutons, the variationof magmatic isotopic composition in the five intrusive suites,in space and time, can be clearly discerned using the ¹⁸O/¹⁶Ovariation in phases such as quartz and sphene that are resistantto sub-solidus exchange. A procedure for analyzing ¹⁸O/¹⁶O insphene using a laser fluorination technique is described: analysisof samples as small as 0.4 mg (including single crystals ofsphene from granitoids) is possible and provides an effectiveway to estimate the magmatic ¹⁸O value of plutonic igneous rocks.Most sphene and quartz ¹⁸O values vary by 1.0–2.0% withineach of the five main intrusive suites in Transbaikalia (rangingin age from mid-Paleozoic to Mesozoic), and are uniform bothwithin individual plutons and among plutons of the same suiteseparated by tens or hundreds of kilometers. However, each suitehas a unique range in ¹⁸O/¹⁶O, indicating that, on a regionalscale, the magmatic ¹⁸O values of these granitoids decreasedprogressively in 1% decrements from +10 in the earliest groupto +6 in the youngest. This progression was accompanied by increasesin the concentration of elements such as K and Zr, and decreasesin the concentration of elements such as Sr and Ba. These systematicsrequire large scale deep crustal melting and mixing processesto generate the compositional uniformity of individual plutonsand groups over such wide areas, and also a progressive hybridizationof the crust with alkalic, mantle-derived magmas to generatesyenites and granites with progressively lower ¹⁸O values. Thisprocess may be a hallmark of anorogenic granitoid petrogenesisand the intracontinental cratonization process in general, andalso represents an important (though largely cryptic) crustalgrowth mechanism. KEY WORDS: anorogenic granitoids; crustal growth; hybridization; hydrothermal systems; stable isotopes ¹Present address: Galson Sciences Limited, 5 Grosvenor House, Melton Road, Oakham LE15 6AX, UK.     

Basalt and Sediment Geochemistry and Magma Petrogenesis in a Transect from Oceanic Island Arc to Rifted Continental Margin Arc: the Kermadec--Hikurangi Margin, SW Pacific

Sediment mixing and recycling through a subduction zone canbe detected in lead isotopes and trace elements from basaltsand sediments from the Kermadec-Hikurangi Margin volcanic arcsystem and their coupled back-arc basins. Sr, Nd and Pb isotopesfrom the basalts delineate relatively simple, almost overlapping,arrays between back-arc basin basalts of the Havre Trough-NgatoroBasin (⁸⁷Sr/⁸⁶Sr = 0.70255; _Nd=+9.3; ²⁰⁶Pb/²⁰⁴Pb = 18.52; ²⁰⁸Pb/²⁰⁴Pb= 38.18), island arc basalts from the Kermadec Arc togetherwith basalts from Taupo Volcanic Zone (⁸⁷Sr/⁸⁶Sr 0.7042; _Nd= +5; ²⁰⁶Pb/²⁰⁴Pb= 18.81; ²⁰⁸Pb/²⁰⁴Pb = 38.61), and sedimentsderived from New Zealand's Mesozoic (Torlesse) basement (⁸⁷Sr/⁸⁶Sr 0.715; _Nd —4; ²⁰⁶Pb/²⁰⁴Pb 18.86; ²⁰⁸Pb/²⁰⁴Pb 38.8).Basalts from the arc front volcanoes have high Cs, Rb, Ba, Th,U and K, and generally high but variable Ba/La, Ba/Nb ratios,characteristic of subduction-related magmas, relative to typicaloceanic basalts. These signatures are diluted in the back-arcbasins, which are more like mid-ocean ridge basalts. Strongchemical correlations in plots of SiO₂ vs CaO and loss on ignitionfor the sediments (finegrained muds) are consistent with mixingbetween detrital and biogenic (carbonate-rich) components. Otherdata, such as Zr vs CaO, are consistent with the detrital componentcomprising a mixture of arc- and continent-derived fractions.In chondrite-normalized diagrams, most of the sediments havelight rare earth element enriched patterns, and all have negativeEu anomalies. The multielement diagrams have negative spikesat Nb, P and Ti and distinctive enrichments in the large ionlithophile elements and Pb relative to mantle. Isotopic measurementsof Pb, Sr and Nd reveal restricted fields of Pb isotopes butwide variation in Nd and Sr relative to other sediments fromthe Pacific Basin. Rare K-rich basalts from Clark Volcano towardthe southern end of the oceanic Kermadec Island Arc show unusualand primitive characteristics ( 2% K₂O at 50% SiO₂, Ba 600p.p.m., 9–10% MgO and Ni > 100 p.p.m.) but have highlyradiogenic Sr, Nd and Pb isotopes, similar to those of basaltsfrom the continental Taupo Volcanic Zone. These oceanic islandarc basalts cannot have inherited their isotope signatures throughcrustal contamination or assimilation—fractional crystallizationtype processes, and this leads us to conclude that source processesvia bulk sediment mixing, fluid and/or melt transfer or somecombination of these are responsible. Although our results showclear chemical gradients from oceanic island arc to continentalmargin arc settings (Kermadec Arc to Taupo Volcanic Zone), overlapbetween the data from the oceanic and continental sectors suggeststhat the lithospheric (crustal contamination) effect may beminimal relative to that of sediment subduction. Indeed, itis possible to account for the chemical changes by a decreasenorthward in the sediment flux into the zone of magma genesis.This model receives support from recent sediment dispersal studiesin the Southern Ocean which indicate that a strong bottom current(Deep Western Boundary Current) flows northward along the easterncontinental margin of New Zealand and sweeps continental derivedsediment into the sediment-starved oceanic trench system. Thetrace element and isotopic signatures of the continental derivedcomponent of this sediment are readily distinguished, but alsodiluted in a south to north direction along the plate boundary. KEY WORDS: subduction zone basalts; sediments; Sr-, Nd-, Pb-isotopes; trace elements ^*Present address: School of Earth Sciences, University of Melbourne, Parkville, Vic. 3052, Australia.